In floor cleaning machines which generally operate with a circular brush rotating about one or two horizontal axes, said brush is driven via a drive disc by a drive motor by means of a flat or circular belt as power transmission. In order to avoid too high a slip on this belt, it must be provided with an adequate prestressing. This results in an additional bearing loading which is greater than the one occuring by the function of the brush. The prestressing also impairs the operation life of the belt. The operation life of the belt is even further shortened by the fact that in not seldom cases of blocking of the rotating brush, e.g. as a result of rug corners clamped in between the brush and the housing or other objects clamped in in this way, the belt slips on the drive disc.
In order to avoid these disadvantages, in other designs, a positive mechanical power transmission is used by means of a gear belt. This, however, can also be destructed in the event the machine is equipped with a strong drive motor and the brush is stopped by blocking.
Thus, means are required which interrupt the power transmission in case of a too high load or even in case of a blocking of the brush. In this regard, a structure is known comprising a heat responsive friction clutch for the protection of the power transmission member. The required friction forces are produced by heat-responsive pressure-engagement springs, so-called bi-metallic springs.
The friction heat generated by blocking causes a deformation of the bi-metallic springs as a result of the material-specific properties thereof and thus a relaxing of the pressure engagement forces, the friction force and the friction heat thereby decreasing (cf. German patent specification No. 2,312,636).
The disadvantage of this known design is the stringent wear of the friction liners provided which continuously slip--initially more and later less--on the countersurface and in doing so generate heat. This likewise results in a reduction of the transmittable torque, a functional reduction of the brush and--as a result of heating up--a drying of the bearings which may then seize.
The same disadvantage is inherent to the safety clutch according to U.S. patent specification No. 3,228,209. The dogs slide on one another in overload conditions and in doing so generate both noise and also troublesome heat. Also, detrimental wear occurs.